In a manufacturing process of various semiconductor devices, a silicon oxide film, such as SiO2, is formed as an insulating film, e.g., a gate insulating film of a transistor. In order to form such a silicon oxide film, a thermal oxidation process using an oxidation furnace or a rapid thermal process (RTP) apparatus, and a plasma oxidation process using a plasma processing apparatus are employed. In a wet oxidation process using an oxidation furnace, which is one of the thermal oxidation processes, a silicon substrate is heated to 800° C. or more and exposed to an oxidizing atmosphere of water vapor (H2O) by using a water vapor generator (WVG), which generates vapor (H2O) through the combustion of oxygen and hydrogen, thereby oxidizing a surface of the silicon substrate to form a silicon oxide film.
On the other hand, in the plasma oxidation process, there is proposed a method, in which a silicon oxide film is formed by using a plasma processing apparatus introducing a microwave into a processing chamber through a radial line slot antenna to generate a plasma (see, e.g., Japanese Patent Laid-open Application No. 2001-160555).
The thermal oxidation process enables the formation of a silicon oxide film of good quality. On the contrary, the plasma oxidation process causes problems, which are not generated in the thermal oxidation process. Such problems may include variation in the film quality of the silicon oxide film, for example, deviation from the stoichiometric ratio in the amounts of Si and O in the silicon oxide film, and a large amount of defects in Si—O bonds due to Si dangling bonds in the film.
Further, in the plasma oxidation process, when a pattern having lines and spaces formed on the surface of an object to be processed includes sparse and dense portions having different pattern density, the silicon oxide film is formed at different rates at different portions, and it is difficult to form the silicon oxide film having a uniform thickness. If the thickness of silicon oxide film varies depending on the pattern density, it may deteriorate the reliability of a semiconductor device using the silicon oxide film as an insulating film.